Heat-treated deformed steel wire and method and apparatus for manufacturing the same

ABSTRACT

Arranged in tandem are a pre-heating unit  6 , a four-side hot-rolling mill  7  and a quenching/cooling unit  8 . A starting material W 1  is rapidly heated to a hot-rolling temperature using the unit  6  and hot-rolled into a modified cross-section steel wire W 2  assuming a predetermined shape in cross section using the mill  7 . The wire is then quenched under tension using the unit  8  immediately after its hot-rolling operation. The tension is applied to the wire by a pinch roll  9  driven by a motor  9   a , which is combined with a torque converter. Consequently, there is no variation in tension, and no variation in cross-sectional dimension in the wire. Disposed behind the unit  8  in tandem is an in-line post-heating coil  10  to realize a continuous tempering operation, so that the wire free from any small-radius bend and having been heat-treated, can be produced through a substantially single process.

TECHNICAL FIELD

[0001] The present invention relates to a modified cross-section steelwire and its production, wherein the steel wire, which has beensubjected to a heat treatment, is for use in coil springs or the likeand assumes, in cross section, a modified shape such as a square shape,a rectangular shape or a nearly trapezoidal shape.

BACKGROUND ART

[0002] Heretofore, it has been known to provide a coil spring, which isconstructed of, for example, a modified cross-section steel wire andproduced through a wire drawing process. In this drawing process, astarting material, i.e., a rolled steel bar assuming a circular shape incross section is drawn to form such a modified cross-section steel wire,as shown in FIG. 5. Such drawn steel wire is then subjected to aquenching treatment which is followed by a tempering treatment such asan oil tempering treatment and the like. After that, as shown in aprocess shown in FIG. 5(a), the steel wire is coiled to form a coilspring, or further subjected to another process shown in FIG. 5(b) aftercompletion of such coiling process, in which another process the coilspring thus formed is subjected to a quenching and a tempering treatmentusing an electric furnace or the like.

DISCLOSURE OF THE INVENTION

[0003] However, since a conventional method for manufacturing a coilspring requires two process steps comprising a wire drawing step fordrawing a modified cross-section wire or the like and a heat treatmentstep including a quenching and a tempering treatment, the conventionalmethod suffers from the large number of its necessary process steps,which makes it difficult for the conventional method to reducemanufacturing costs.

[0004] Further, in the method for coiling the steel wire having beenheat-treated as shown in FIG. 5(a), when the wire to be coiled has awave, a twist and/or a slight bend in shape, the wire thus coiled isdeformed in shape, thereby causing a decrease in yield of products. Inthis respect, however, even when the wire having been treated by theconventional method is subjected to a quenching and a temperingtreatment through heating in the furnace, it is difficult to improve theproducts in quality. Due to this, a need still exists in the art for asteel wire-to-be-quenched which is improved in smoothness.

[0005] On the other hand, in order to obtain such a steel wire improvedin smoothness, there is proposed a method for heating and quenching thesteel wire under a proper tension. However, when the steel wire isheated to a high temperature, the thus heated steel wire is easilyelongated in its longitudinal direction under tension to reduce itscross-sectional area. In other words, when the tension applied to thesteel wire varies, the cross-sectional area of the steel wire alsovaries. This is a problem inherent in the art.

[0006] Consequently, it is an object of the present invention to providea method for producing a modified cross-section steel wire, wherein astarting material, i.e., a steel wire assuming a circular shape in crosssection is continuously hot-rolled into a modified cross-section steelwire and simultaneously subjected to a quenching treatment through aheat treatment, whereby the modified cross-section steel wire havingbeen heat-treated is produced through a single process step. Inaddition, it is anther object of the present invention to provide amethod for producing a modified cross-section steel wire having beenheat-treated, wherein the steel wire is quenched under a stable tensionfree from any variations, which results in the steel wire excellent bothin accuracy of dimensions and in smoothness of shape. Particularly, thepresent invention is useful for production of a coil spring constructedof a high-strength steel wire. Further, the present invention is alsouseful for production of various types of a modified cross-section steelwire serving as a square cross-section torsion bar and the like, whichhas a modified cross section and must be smooth in shape.

[0007] In order to accomplish the above objects of the presentinvention, a modified cross-section steel wire of the present inventionand a method of its production are characterized in that: a startingmaterial of the steel wire is rapidly heated to a hot-rollingtemperature in a short period of time using an induction heating meansand/or directly using a resistance heating means, hot-rolled into amodified cross-section steel wire having a predetermined cross sectionand then quenched immediately after completion of its hot-rollingoperation.

[0008] In other words, when the starting material of the steel wire israpidly heated in a short period of time by induction heating and thelike, it is possible to avoid occurrence of a problem of decarbonizationwhich often occurs in heating in an ordinary furnace and the like. It isalso known that the induction heating or the like is effective inobtaining large quenching effects. When the steel wire is quenchedimmediately after completion of its hot-rolling operation, it ispossible to avoid “grain growth” in the steel wire and also avoid Ar3transformation in a surface layer portion of the steel wire. Thispermits the surface of the steel wire to be rapidly cooled to preventthe precipitation of ferrite, which makes it possible to quench thesteel wire up to its surface layer portion. As is clear from the abovedescription, by quenching the steel wire immediately after completion ofits hot-rolling operation, it is possible to perform the hot-rolling andthe quenching operation of the modified cross-section steel wire througha single process step. Further, the quenching operation makes itpossible for the steel wire to be improved in strength relative to thatsubjected to a conventional quenching operation.

[0009] When the four-side hot-rolling mill is used to perform the abovehot-rolling operation, it is possible to obtain the modifiedcross-section steel wire in an easy manner using a small number of millstands.

[0010] Preferably, the modified cross-section steel wire is produced by:rapidly heating the starting material of the steel wire at a heatingrate of equal to or more than 100° C./s to a temperature of equal to ormore than the Ac3 point in a short period of time; hot-rolling the thusheated starting material of the steel wire at a temperature of equal toor more than the Ar3 point into the modified cross-section steel wirehaving a predetermined cross section; and, quenching the thus hot-rolledmodified cross-section steel wire at a temperature of equal to or morethan the Ar1 point using the quenching/cooling means disposed adjacentto the hot-rolling mill in tandem therewith, immediately aftercompletion of its hot-rolling operation. Preferably, thisquenching/cooling means is disposed as close as possible to thehot-rolling mill. This makes it possible to perform the quenchingoperation of the steel wire immediately after completion of itshot-rolling operation, which prevents a so-called “grain growth” fromoccurring in the steel wire thus hot-rolled and therefore makes itpossible to improve the thus immediately-quenched hot-rolled steel wirein hardness.

[0011] As described above, by hot-rolling the starting material of thesteel wire into the modified cross-section steel wire using thehot-rolling mill, it is possible to obtain a spring steel wire which isfree from any scratch and excellent in smoothness in its surface incontrast with a corresponding wire product produced through aconventional wire drawing process. Further, as for high alloy steels,though it is difficult to draw the high alloy steels since the highalloy steels are large in work hardening, the present invention makes itpossible to draw the high alloy steels into a modified cross-sectionsteel wire in an easy manner.

[0012] As for the above-mentioned quenching operation, in order toimprove the hot-rolled steel wire in straightness, it is preferable toquench the modified cross-section steel wire under tension immediatelyafter completion of the hot-rolling operation of the starting materialof the steel wire. Further, in order to reduce variations in dimensionsof the steel wire resulted from variations in tension applied to thesteel wire, it is preferable to employ any one of a pinch roll, a wiretake-up reel and a capstan unit which are driven by a drive unitcombined with a torque converter capable of absorbing any variations intension of the steel wire.

[0013] As described above, in performing a coiling operation of thesteel wire to form a coil spring, a bend with a predetermined largeradius of the steel wire does not affect the properties of the product.In contrast with this, when the steel wire to be coiled has a wave, atwist and/or a bend with a small radius in shape, the wire product thuscoiled is deformed in shape, thereby causing a decrease in yield of theproducts. In the method of the present invention, since the quenchingoperation of the steel wire is performed under tension applied to thesteel wire immediately after completion of the hot-rolling operation, itis possible to obtain the spring steel wire which has been quenched andis excellent in straightness. This makes it possible to improve the thusquenched steel wire or product in yield when the product is coiled intoa coil spring. Incidentally, in addition to the above use (coil spring),also in other uses for the steel wire such as a torsion bar and the likeassuming a straight shape, it is possible to considerably reduce thenumber of necessary process steps such as a process step forstraightening the steel wire and the like. This leads to a considerablereduction of the manufacturing cost.

[0014] However, since the steel wire having been heated to a hightemperature for performing the quenching operation thereof is low instrength, there is a fear that the dimensions of the steel wire vary incross section when the tension applied to the steel wire varies.Consequently, the present invention is characterized in that: using apinch roll, a wire take-up reel and a capstan unit which are driven by adrive unit combined with a torque converter, any variations in tensionin the steel wire is absorbed by such a torque converter, so that thetension of the steel wire is maintained at a predetermined level whenthe steel wire is quenched. This makes it possible for the presentinvention to obtain the steel wire which has been quenched, is free fromvariations in its cross section and excellent in dimension accuracy.Particularly, as for a modified cross section steel having a smalldiameter, such a small-diameter steel wire is large in variations incross section when directly subjected to tension. However, this problemof variations in cross section of the steel wire can be solved in thepresent invention by using the torque converter through which a tensionis applied to such a small-diameter steel wire when the steel wire isquenched here, the term “drive unit combined with the torque converter”means not only a combination system constructed of the drive unit andthe torque converter, but also any system for driving the steel wirethrough the torque converter.

[0015] In the present invention, it is preferable to continuouslyperform the quenching and the tempering operation of the steel wire byperforming the quenching operation which is followed by an inductionheating operation. In this induction heating process, the steel wire isheated to a tempering temperature thereof. After the steel wire reachessuch a tempering temperature thereof, the steel wire is tempered.Preferably, such a tempering operation of the steel wire is continuouslyperformed using the induction heating coil, wherein the inductionheating coil is disposed in a downstream side of the quenching means orunit in a tandem manner.

[0016] In general, the method of the present invention is effective inproducing the modified cross-section steel wire assuming, in crosssection, either a square shape, a rectangular shape or a nearlytrapezoidal shape. It is possible for the method of the presentinvention to produce the modified cross-section steel wire in an easymanner, wherein the steel wire has been quenched and has a tensilestrength of equal to or more than 1600 N/mm².

[0017] An apparatus of the present invention for producing a modifiedcross-section steel wire having been heat-treated, comprises: an in-linepre-heating means or unit for heating a starting material of themodified cross-section steel wire to a hot-rolling temperature using aninduction heating means or unit or directly using a resistance heatingmeans or unit; a hot-rolling means or unit for hot-rolling the startingmaterial into a modified cross-sectional steel wire having apredetermined cross section; a quenching means or unit for quenching thethus hot-rolled modified cross-section steel wire under tension appliedthereto; and, a tension applying means or unit for applying a tension tothe modified cross-section steel wire thus hot-rolled, wherein all themeans or units are arranged in tandem.

[0018] Preferably, the hot-rolling means or unit is constructed of afour-side hot-rolling mill. Preferably, the tension applying means orunit is constructed of either a pinch roll, a wire take-up reel or acapstan unit driven by a drive unit provided with a torque converter forabsorbing variations in tension appearing in the steel wire. Disposed ina downstream side of the quenching/cooling means or unit in tandem is anpost-heating means or unit for heating the steel wire to a temperingtemperature, which makes it possible for the apparatus of the presentinvention to continuously perform the quenching operation and thetempering operation of the steel wire.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] [FIG. 1]: a view illustrating in construction an embodiment ofthe apparatus of the present invention for producing the modifiedcross-section steel wire having been heat-treated;

[0020] [FIG. 2]: a view illustrating in configuration the cross sectionof an embodiment of the modified cross-section steel wire having beenheat-treated, according to the present invention;

[0021] [FIG. 3]: a view illustrating rolls of the four-side hot-rollingmill in the embodiment of the present invention;

[0022] [FIG. 4]: a flow diagram illustrating the process steps of theembodiment of the present invention for producing the modifiedcross-section steel wire having been heat-treated;

[0023] [FIG. 5]: a flow diagram illustrating a method for producing acoil spring constructed of a conventional type modified cross-sectionsteel wire having been heat-treated;

[0024] [FIG. 6]: a Table illustrating the chemical composition of therolled steel wire of the embodiment of the present invention;

[0025] [FIG. 7]: a Table illustrating the particulars of the modifiedcross-section steel wire having been heat-treated in the embodiment ofthe present invention;

[0026] [FIG. 8]: a Table illustrating temperatures measured in variouspositions of the apparatus in the embodiment of the present invention;and

[0027] [FIG. 9]: a Table illustrating one of examples of the mechanicalproperties of the modified cross-section steel wire having beenheat-treated in the embodiment of the present invention.

DESCRIPTION OF THE REFERENCE CHARACTERS

[0028]1: an upper and a lower horizontal roll; 2: a left and a rightvertical roll; 6: an induction heating coil (i.e., an in-linepre-heating means or unit); 7: a four-side hot-rolling mill; 8: awater-cooled jacket (i.e., a quenching means or unit); 9: a pinchroll(i.e., a tensioning means); 9 a: a motor combined with a torqueconverter; 10: an in-tempering operation heating coil; 11: a wiretake-up reel; W1: a starting material of a steel wire; and, W2: amodified cross-section steel wire.

BEST MODE FOR CARRYING OUT THE INVENTION

[0029] With reference to an embodiment of the present invention shown inthe accompanying drawings, present invention will be described in aconcrete manner. FIG. 1 is a view illustrating the concept of thepresent invention in construction with the use of an embodiment of anapparatus of the present invention for producing the modifiedcross-section steel wire having been heat-treated. FIG. 2 is a viewillustrating in configuration the cross section of an embodiment of themodified cross-section steel wire having been heat-treated, wherein thesteel wire is used in the embodiment of such an apparatus of the presentinvention. FIG. 3 is a view illustrating, in configuration, each ofrolls of the four-side hot-rolling mill (i.e., a hot-rolling means orunit) used in the hot-rolling operation of the steel wire according tothe present invention. FIG. 4 is a flow diagram illustrating the processsteps required in producing the modified cross-section steel wire havingbeen heat-treated according to the embodiment of the present invention.

[0030] In FIG. 1, arranged in tandem from the left (i.e., an upstreamside of FIG. 1) to the right (i.e., a downstream side of FIG. 1) are: anin-line pre-heating coil (i.e., an in-line pre-heating means or unit) 6;a four-side hot-rolling mill (i.e., a four-side hot-rolling means orunit) 7; a water-cooled jacket (i.e., a quenching/cooling means or unit)8; a pinch roll (i.e., a tension applying means or unit) 9; and, anin-line post-heating coil (i.e., an in-line post-heating means of unit)10; and, a wire take-up reel 11. The pinch roll 9 is driven by a motor 9a which is combined with a torque converter provided with aspeed-reduction unit.

[0031] As shown in FIG. 2, a modified cross-section steel wire W2 of anembodiment of the present invention has been heat-treated and assumes anearly trapezoidal shape in cross section. Used in a hot-rollingoperation of such a modified cross-section steel wire W2 is thefour-side hot-rolling mill 7. As shown in FIG. 3, the four-sidehot-rolling mill 7 is constructed of: a pair of upper and lowerhorizontal rolls 1, 1 adjacent to upper and lower non-parallel sides ofthe nearly trapezoidal cross section of the steel wire W2; and, a pairof left and right vertical rolls 2, 2 adjacent to upper and lowerparallel sides of the nearly trapezoidal cross section of the steel wireW2.

[0032] Now, referring to FIGS. 1 and 4, a method for producing themodified cross-section steel wire W2 will be described. First of all, ina preparatory stage of the method, a piece of rolled steel having apredetermined diameter is descaled. After that, the piece of rolledsteel thus descaled is drawn into a starting material W1 of the steelwire W2 while keeping its round shape in cross section. Such a startingmaterial W2 is then delivered to the in-hot-rolling process heating coil6 from a pay-off reel (not shown). The starting material W1 isinduction-heated using the in-hot-rolling process heating coil 6, andthen delivered to the four-side hot-rolling mill 7, in which thestarting material W1 is hot-rolled into the steel wire W2 assuming anearly trapezoidal shape in cross section, as shown in FIG. 2.Immediately after completion of such a hot-rolling operation, the steelwire W2 thus hot-rolled is quenched using the water-cooled jacket 8. Atthis time, since the pinch roll 9 is rotatably driven at a speedslightly higher than a traveling speed of the modified cross-sectionsteel wire W2 measured in a downstream side of the hot-rolling mill 7,the steel wire W2 is delivered from the mill 7 under tension applied bythe pinch roll 9 to the following quenching stage in which the quenchingoperation of the steel wire W2 is performed. As described above, byquenching the steel wire W2 under tension, it is possible to obtain thesteel wire W2 excellent in straightness and having been quenched.Further, since the pinch roll 9 is driven by the motor 9 a combined withthe torque converter provided with the speed-reduction unit, there is nofear that the steel wire W2 is exposed to excessive tension.Consequently, it is possible to prevent the steel wire W2 from varyingin dimension. After that, the steel wire W2 thus quenched is heated to atempering temperature using the in-tempering operation heating coil 10and then tempered. Subsequent to this tempering operation, the steelwire W2 thus tempered is taken up by the wire take-up reel 11. Then, inthe production of a coil spring, the modified cross-section steel wirethus taken up by the reel 11 is coiled to assume a predetermined shape.

[0033] In other words, as shown in FIG. 5, a conventional method isconstructed of two steps comprising the-steps of: drawing a startingmaterial into a modified cross-section steel wire; and, quenching andtempering the steel wire thus drawn. In contrast with this, in thepresent invention, both the step of drawing the starting material intothe modified cross-section steel wire and the step of quenching andtempering the steel wire described in the conventional method areperformed in a single step of the method of the present invention.

[0034] Incidentally, in this embodiment of the present invention, thoughthe steel wire is subjected to a tension applied by the pinch-roll, itis possible to replace the pinch roll with a capstan unit. Further, itis also possible to apply a tension to the steel wire by means of thewire take-up reel. In case that the tension is applied to the steel wireby means of the capstan unit and/or the wire take-up reel, a motorcombined with a torque converter is used to drive the capstan unitand/or the wire take-up reel. As for in-line heating performed in thehot-rolling process, it is possible to realize such in-line heatingthrough induction heating and/or direct resistance heating.

EXAMPLES

[0035] Hereinbelow, an example 1 of the modified cross-section steelwire W2 according to the present invention will be described. FIG. 6shows the chemical composition of the example 1 of the steel wire W2having been rot-rolled.

[0036] A piece of hot-rolled steel was drawn into a round bar, i.e., astarting material W1 of the modified cross-section steel wire W2 of thepresent invention, wherein the steel wire W2 assumes a nearlytrapezoidal shape in cross section, as shown in FIG. 2. FIG. 7 is aTable, which illustrates the particulars of the modified cross-sectionsteel wire W2, and, more particularly illustrates the relationshipbetween: dimensions in cross section of the modified cross-section steelwire W2 of the example 1 which has been hot-rolled and assumes such anearly trapezoidal shape in cross section; and, a diameter of thecorresponding starting material W1 of the steel wire W2. In this example1, a so-called “reduction of area”, i.e., the percentage of decrease incross sectional area of the drawn bar or wire was equal to approximately0.9. Each of a plurality of the steel wires W2 each assuming a nearlytrapezoidal shape but varying in dimension was subsequently subjected toa quenching and a tempering operation under tension. As for the modifiedcross-section steel wires W2 which is small in diameter such as ones ofthe 40 series, the 30 series and the 25 series, a quenching operationperformed under tension according to the conventional method failed toproduce a product constant in cross section since these small-diametersteel wires W2 were large in variations in cross sectional dimension. Incontrast with this, in the method of the present invention, since atension was applied to the steel wire W2 by means of the pinch roll 9combined with the torque converter, it was possible to quench suchsmall-diameter modified cross-section steel wire W2 under tensionwithout a fear that the steel wire or product W2 varies in diameter.

[0037] Of a plurality of products W2 shown in FIG. 7, one of an L-60type was employed as an example 1, which will be described hereinbelow.The product of the L-60 type assumed a nearly trapezoidal shape in crosssection such as one shown in FIG. 2, wherein the shape had a height of14.40 mm, a length in lower side of 7.10 mm, and a length in upper sideof 4.70 mm. On the other hand, a round bar having a diameter of 10.80 mmwas used as the starting material

[0038] W1 of the steel wire W2, which had been hot-rolled and assumedthe above shape in cross section. A so-called “reduction of area”, i.e.,the percentage of decrease in cross-sectional area of the round bar orstating material W1 of the steel wire W2 during its hot-rollingoperation was 0.9059. In this example 1, variations in dimension incross section of the modified cross-section steel wire W2 having beenheat-treated was within a range of a value of equal to or less than 0.05mm, which makes it possible to obtain the heat-treated modifiedcross-section steel wire W2 excellent in precision.

[0039] The heat treatment of the steel wire W2 was conducted under thefollowing conditions: namely, the round bar or starting material W1 washeated up to its rolling temperature in a period of 7 seconds; and, thesteel wire W2 was delivered from an exit side of the apparatus of thepresent invention at a traveling speed of 100 mm/sec. As shown in FIG.1, a plurality of temperatures at which the individual heat treatmentswere conducted in the example 1 were measured at a plurality ofpositions comprising: an exit side {circle over (1)} of the in-linepre-heating coil 6; a soaking position, i.e., a downstream side {circleover (2)} of the in-line pre-heating coil 6, in which side {circle over(2)} the starting material. W1 thus heated was stabilized in temperatureas a whole; an inlet side {circle over (3)} of the hot-rolling mill 7;an exit side {circle over (4)} of the hot-rolling mill 7; an inlet side{circle over (5)} of the water-cooled jacket 8; and, an exit side{circle over (6)} of the in-line post-heating coil 10. FIG. 8 showstemperatures measured in the above positions of the apparatus of thepresent invention.

[0040] In other words, the round bar or starting material w1 of thesteel wire w2 was heated to a temperature of approximately 1024° C.which is equal to or more than the Ac3 point by using the in-linepre-heating coil 6 for heating the round bar w1 prior to its hot-rollingoperation. As a result, the round bar W1 had its atoms rearranged into aso-called “γ(gamma)” form. After that, by means of the four-sidehot-rolling mill 7, the round bar W1 was hot-rolled into the modifiedcross-section steel wire W2 at a temperature of equal to or more thanthe Ar3 point ranging from 850° C. to 760° C. Immediately aftercompletion of the hot-rolling operation, the steel wire W2 was quenchedat a temperature of 740° C. which is equal to or more than the Ar1point. This made it possible for the present invention to obtain thesteel wire W2 having a fine structure of martensite excellent instrength. The thus obtained steel wire W2 was heated again up to atempering temperature of approximately 500° C. and then tempered.

[0041]FIG. 9 shows the mechanical properties of the modifiedcross-section steel wire W2 having been heat-treated in the example 1.In other words, the modified cross-section steel wire W2 of the L-60type having been heat-treated under the above conditions had a largetensile strength of equal to or more than 1600 N/mm² and a largeelongating rate of equal to or more than 10%.

[0042] In addition, the modified cross-section steel wire W2 thusheat-treated was excellent in straightness and free from anysmall-radius bend. Consequently, in this example 1, the modifiedcross-section steel wire W2 thus heat-treated was formed into a coilspring. When a conventional type of the modified cross-section steelwire is formed into a coil spring or product, its product defective rateoften reaches a value of 10%. In contrast with this, the modifiedcross-section steel wire W2 thus heat-treated according to the presentinvention and formed into a coil spring had a product defective rate ofsubstantially 0%.

[0043] Although the other types of heat-treated modified cross-sectionsteel wires W2 shown in FIG. 7 were hot-rolled, any one of them wasexcellent in accuracy in dimension since they varied in cross-sectionaldimension within a range of equal to or less than 0.05 mm. In additionto the above, the mechanical properties of the other types of the steelwires W2 are also shown in FIG. 7, as an example. As is clear from FIG.7, the other types of the steel wires W2 were also excellent in each ofstraightness, tensile strength and elongation rate, as is in theexample 1. Consequently, all the other types of the modifiedcross-section steel wires W2 of the present invention were found to besubstantially free from any defect since any one of these steel wires W2of the present invention had a product defective rate of substantially0% when formed into a coil spring, i.e., product, as is in the example1.

[0044] In the above examples including the example 1, the round bar W1was hot-rolled into the modified cross-section steel wire W2 and furtherformed into the coil spring, which was estimated and confirmed in effectof the present invention. Further, it is possible to apply the presentinvention to the steel wire W2 having, in cross section, not only thenearly trapezoidal shape but also other shapes such as a square shape ora rectangular shape in cross section of the steel wire W2. Inapplication, the present invention is also applicable to not only thecoil spring, but also other types of wire products such as a torsion barand like linear products since the steel wire W2 of the presentinvention is excellent in straightness, which permits the method of thepresent invention to reduce the number of necessary process steps of themethod of the present invention for producing such linear products eachconstructed of the modified cross-section steel wire W2 having beenheat-treated, so that the production yield in the method of the presentinvention can be improved greatly. It is also possible to apply thepresent invention to various application fields other than the wiresprings.

[0045] Industrial Applicability:

[0046] As described above, according to the method of the presentinvention for producing the modified cross-section steel wire W2 havingbeen heat-treated, it is possible to rapidly heat each of the round barW1 and the steel wire W2 to a predetermined temperature in a very shortperiod of time by using induction heating and the like. Consequently, itis possible for the method of the present invention to avoid occurrenceof a problem of decarbonization which often occurs in heating in anordinary furnace and the like. Further, in the method of the presentinvention, since the hot-rolled steel wire W2 of the present inventionis quenched immediately after completion of its hot-rolling operation,it is possible to produce the hot-rolled modified cross-section steelwire W2 through a single process step. The above quenching operation isfollowed by a rapid heating and a tempering process step, which makes itpossible to produce the modified cross-section steel wire W2 excellentin strength.

[0047] In the method of the present invention: the round bar W1 israpidly heated to a tempering temperature of equal to or more than theAc3 point; the thus heated round bar W1 is then hot-rolled into themodified cross-section steel wire W2 at a rolling temperature of equalto or more than the Ar3 point; immediately after completion of suchhot-rolling operation, the thus hot-rolled steel wire W2 is directlyquenched at a temperature of equal to or more than the Ar1 point byusing the quenching/cooling means or unit 8, so that the steel wire W2having a tensile strength of equal to or more than 1600 N/mm² isproduced. Further, in the apparatus of the present invention, thein-line post-heating means or unit 10 for heating the thus quenchedsteel wire W2 to a tempering temperature is disposed in a downstreamside of the quenching/cooling means or unit 8 in tandem, which makes itpossible to have the quenching and the tempering operation conducted ina continuous manner in the method of the present invention.

[0048] Further, in the method of the present invention, since the roundbar W2 is hot-rolled into the modified cross-section steel wire W2 usingthe hot-rolling mill 7, it is possible to reduce the number of surfacedefects of the product such as a scratch on the surface of the productand like defects, which makes it possible to produce the product ormodified cross-section steel wire W2 excellent in smoothness in surface.In contrast with this, a product produced through a conventionalcold-drawing process is poor in smoothness in surface. Further, it isalso possible for the method of the present invention to produce aproduct made of high alloy steel which is difficult to be cold-drawnthrough the conventional process.

[0049] Still further, in the method of the present invention, since thesteel wire W2 having been hot-rolled is subjected to the quenchingprocess under tension, the modified cross-section steel wire W2 of thepresent invention having been heat-treated is free from any defect suchas a wave, a twist and/or a slight or small-radius bend in shape, andtherefore excellent in straightness in shape. The tension is applied tothe steel wire W2 in the quenching process by means of the pinch roll 9and/or the wire-take-up reel and/or the capstan unit, any variation intension does not appear in the steel wire W2. Therefore, the modifiedcross-section steel wire or product of the present invention issubstantially free from any variation in its cross-sectional dimension.This is one of characterized features of the present invention. Theeffect of the present invention is large particularly as to one of themodified cross-section steel wires W2, which one has a small diameter.Due to this, an effective quenching process for quenching thesmall-diameter ones of the modified cross-section steel wires W2 isrealized by the method of the present invention. In contrast with this,such an effective quenching process is not realized in the prior art.

[0050] Although the pre-heating of the round bar W1 is conducted byinduction heating, it is also possible for the method of the presentinvention to employ any other heating process such as a directresistance heating process and the like. It is also possible to employboth the induction heating and the direct resistance heating process atthe same time. Further, though a single stand of the four-sidehot-rolling mill 7 is employed in the above embodiment of the method ofthe present invention, it is also possible for the method of the presentinvention to employ two or more stands of the hot-rolling mills 7. Stillfurther, though the modified cross-section steel wire W2 having beenheat-treated assumes a nearly trapezoidal shape in cross section in theembodiment of the present invention, it is also possible for the steelwire W2 of the present invention to assume any other shape such as asquare shape, a rectangular shape, or like shape in cross section.

[0051] As described above, according to the modified cross-section steelwire W2 having been heat-treated and the method of the presentinvention, it is possible to produce the heat-treated modifiedcross-section steel wire W2 of the present invention excellent in bothstraightness and strength through a single process step. Due to this, itis possible for the present invention to produce the product or coilspring which is substantially free from any defect such as ones resultedfrom the small-radius bend of the steel wire W2,-which makes it possibleto have the product defective rate reduced to a level of substantially0%, and therefore possible to considerably reduce the manufacturing costof the product or coil spring which is constructed of the modifiedcross-section steel wire W2. Particularly, the present invention iseffective in production of the small-diameter modified cross-sectionsteel wire W2 having been heat-treated. Further, it is also possible toapply the present invention to any other application field such as oneof a torsion bar constructed of the modified cross-section steel wireW2.

1. A modified cross-section steel wire having been heat-treated,characterized in that: a starting material of said steel wire is rapidlyheated to a hot-rolling temperature in a short period of time using aninduction heating means and/or directly using a resistance heatingmeans, hot-rolled into a modified cross-section steel wire having apredetermined shape in cross section and then quenched immediately aftercompletion of its hot-rolling operation.
 2. The modified cross-sectionsteel wire having been heat-treated as set forth in claim 1, whereinsaid modified cross section steel wire assumes, in cross section, eithera square shape, a rectangular shape or a nearly trapezoidal shape.
 3. Amethod for producing a modified cross-section steel wire having beenheat-treated, comprising the steps of: rapidly heating a startingmaterial of said steel wire to a hot-rolling temperature in a shortperiod of time using an induction heating means and/or directly using aresistance heating means; hot-rolling said starting material thus heatedinto a modified cross-section steel wire having a predetermined shape incross section; and, quenching said modified cross-section steel wireimmediately after completion of its hot-rolling operation.
 4. The methodfor producing the modified cross-section steel wire having beenheat-treated as set forth in claim 3, wherein said starting material ishot-rolled using a four-side hot-rolling mill.
 5. The method forproducing the modified cross-section steel wire having been heat-treatedas set forth in claim 3, wherein: said starting material of said steelwire is rapidly heated at a heating rate of equal to or more than 100°C./s to a temperature of equal to or more than the Ac3 point in a shortperiod of time, and then hot-rolled at a temperature of equal to or morethan the Ar3 point into said modified cross-section steel wire havingsaid predetermined shape in cross section using a hot-rolling mill; and,said modified cross section steel wire thus hot-rolled is quenchedimmediately after completion of its hot-rolling operation at atemperature of equal to or more than the Ar1 point using aquenching/cooling means disposed adjacent to said hot-rolling mill in atandem manner.
 6. The method for producing the modified cross-sectionsteel wire having been heat-treated as set forth in claim 3, whereinsaid modified cross-section steel wire is quenched immediately aftercompletion of its hot-rolling operation under tension applied to saidsteel wire.
 7. The method for producing the modified cross-section steelwire having been heat-treated as set forth in claim 6, wherein saidtension applied to said modified cross-section steel wire using either apinch roll, a wire take-up reel or a capstan unit each of which servesas a drive unit, said drive unit being provided with a torque converterfor absorbing variations in tension of said steel wire.
 8. The methodfor producing the modified cross-section steel wire having beenheat-treated as set forth in claim 3, wherein, after completion of thequenching operation of said modified cross-section steel wire, saidsteel wire is continuously heated to a tempering temperature using aninduction heating means to perform a tempering operation of saidmodified cross-section steel wire.
 9. The method for producing themodified cross-section steel wire having been heat-treated as set forthin claim 8, wherein said modified cross-section steel wire having beenheat-treated has a tensile strength of equal to or more than 1600 N/mm².10. The method for producing the modified cross-section steel wirehaving been heat-treated as set forth in any one of claims 3 to 9,wherein said modified cross-section steel wire having been heat-treatedassumes, in cross section, either a square shape, a rectangular shape ora nearly trapezoidal shape.
 11. An apparatus for producing a modifiedcross-section steel wire having been heat-treated, comprising: anin-line pre-heating means for heating a starting material of saidmodified cross-section steel wire to a hot-rolling temperature using aninduction heating means or directly using a resistance heating means; ahot-rolling means for hot-rolling said starting material into a modifiedcross-sectional steel wire having a predetermined shape in crosssection; a quenching means for quenching the thus hot-rolled modifiedcross-section steel wire under tension applied thereto; and, a tensionapplying means for applying a tension to said modified cross-sectionsteel wire thus hot-rolled, wherein all said means are arranged in atandem manner.
 12. The apparatus for producing the modifiedcross-section steel wire having been heat-treated as set forth in claim11, wherein said hot-rolling means is constructed of a four-sidehot-rolling mill.
 13. The apparatus for producing the modifiedcross-section steel wire having been heat-treated as set forth in claim11, wherein said tension applying means is constructed of either a pinchroll, a wire take-up reel or a capstan unit each driven by a drive unitprovided with a torque converter for absorbing variations in tensionappearing in said steel wire.
 14. The apparatus for producing themodified cross-section steel wire having been heat-treated as set forthin claim 11, wherein: disposed behind said quenching means in tandem isan in-line post-heating means for heating said steel wire to a temperingtemperature.